Among Drosophila melanogaster, as among members of other species, individuals vary in their ability to obtain mates. This variability has been shown to have both genetic and nongenetic components. Because differential mating success may profoundly influence the genetic composition of subsequent generations, it is unfortunate that the behavioral basis for individual mating success is not well understood. The principal investigator has developed a comprehensive multivariate analysis of Drosophila courtship in order to compare the behavior of successfully and unsuccessfully courting animals (Markow and Hanson 1981). It is proposed below that this technique be applied to the following questions: 1) What is the relationship between variability in courtship behavior and mating success across a variety of genotypes and treatments? and 2) How do ne mate recognition systems arise during evolution? The first question will be approached by comparing the multivariate courtship profiles (MCPs) of flies of contrasting genotype and treatments known to predictably alter competitive courtship success. The importance of particular courtship features will be assessed. Sensory deficient mutants will be employed to examine the role of each sensory mode for both sexes in determining the outcome of mate choice experiments. In approaching the second question, a unique new mate recognition system which has arisen in laboratory selected phototactic strains of Drosophila will be employed. The roles of sensory mode and genetic architecture underlying the newly evolved male and female behaviors in selected strains will be investigated.